The present invention relates generally to mass spectrometry and the analysis of chemical samples, and more particularly to ionization source chambers and ion beam delivery systems used in mass spectrometry. An apparatus for an ionization source chamber and ion beam delivery system is described for the generation of ions from a sample for subsequent analysis in a mass spectrometer.
The present invention relates in general to ionization source chambers and ion beam delivery systems for use in mass spectrometry, and more particularly to an ionization source chambers and ion beam delivery system having improved flexibility and accessability over prior art sources. The apparatus and method for ionization described herein are enhancements of the techniques that are referred to in the literature relating to mass spectrometry.
Mass spectrometry is an important tool in the analysis of a wide range of chemical compounds. Specifically, mass spectrometers can be used to determine the molecular weight of sample compounds. The analysis of samples by mass spectrometry consists of three main stepsxe2x80x94formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. A variety of means exist in the field of mass spectrometry to perform each of these three functions. The particular combination of means used in a given spectrometer determine the characteristics of that spectrometer.
To mass analyze ions, for example, one might use a magnetic (B) or electrostatic (E) analyzer. Ions passing through a magnetic or electrostatic field will follow a curved path. In a magnetic field the curvature of the path will be indicative of the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. If magnetic and electrostatic analyzers are used consecutively, then both the momentum-to-charge and energy-to-charge ratios of the ions will be known and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers.
Before mass analysis can begin, however, gas phase ions must be formed from sample material. If the sample material is sufficiently volatile, ions may be formed by electron impact (EI) or chemical ionization (CI) of the gas phase sample molecules. For solid samples (e.g. semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Secondary ion mass spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules. As a result, fragile molecules will be fragmented. This fragmentation is undesirable in that information regarding the original composition of the sample xe2x80x94e.g., the molecular weight of sample moleculesxe2x80x94will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res Commoun. 60 (1974) 616). Macfarlane et al. discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules, however, unlike SIMS, the PD process results also in the desorption of larger, more labile speciesxe2x80x94e.g., insulin and other protein molecules.
Lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; or Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter, R. J., Anal. Instrument. 16 (1987) 93. Cotter et al. modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151 and Karas, M.; Hillenkamp, F., Anal. Chem. 60 (1988) 2299). In the MALDI process, an analyte is dissolved in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by the analyte is used to excite the sample. Thus, the matrix is excited directly by the laser, and the excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are then ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. This process, MALDI, is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
Atmospheric pressure ionization (API) includes a number of methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. Very large ions can be formed in this way. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
ESMS was introduced by Yamashita and Fenn (M. Yamashita and J. B. Fenn, J. Phys. Chem. 88, 4671, 1984). To establish this combination of ESI and MS, ions had to be formed at atmospheric pressure, and then introduced into the vacuum system of a mass analyzer via a differentially pumped interface. The combination of ESI and MS afforded scientists the opportunity to mass analyze a wide range of samples. ESMS is now widely used primarily in the analysis of biomolecules (e.g. proteins) and complex organic molecules.
In the intervening years a number of means and methods useful to ESMS and API-MS have been developed. Specifically, much work has focused on sprayers and ionization chambers. In addition to the original electrospray technique, pneumatic assisted electrospray, dual electrospray, and nano electrospray are now also widely available. Pneumatic assisted electrospray (A. P. Bruins, T. R. Covey, and J. D. Henion, Anal. Chem. 59, 2642, 1987) uses nebulizing gas flowing past the tip of the spray needle to assist in the formation of droplets. The nebulization gas assists in the formation of the spray and thereby makes the operation of the ESI easier. Nano electrospray (M. S. Wilm, M. Mann, Int. J. Mass Spectrom. Ion Processes 136, 167, 1994) employs a much smaller diameter needle than the original electrospray. As a result the flow rate of sample to the tip is lower and the droplets in the spray are finer. However, the ion signal provided by nano electrospray in conjunction with MS is essentially the same as with the original electrospray. Nano electrospray is therefore much more sensitive with respect to the amount of material necessary to perform a given analysis.
For example, FIG. 1 depicts a conventional mass spectrometer using an ESI ion source. Ions are produced from sample material in an ionization chamber 4. Sample solution enters the ionization chamber through a spray needle 5, at the end of which the solution is formed into a spray of fine droplets 11. The spray is formed as a result of an electrostatic field applied between the spray needle 5 and a sampling orifice 7. The sampling orifice may be an aperture, capillary, or other similar inlet leading into the vacuum chambers (1,2 and 3) of the mass spectrometer. Electrosprayed droplets evaporate while in the ionization chamber thereby producing gas phase analyte ions. In addition, heated drying gas may be used to assist the evaporation of the droplets. Some of the analyte ions are carried with the gas from the ionization chamber 4 through the sampling orifice 7 and into the vacuum system (comprising vacuum chambers 1, 2 and 3) of the mass spectrometer. With the assistance of electrostatic lenses and/or RF driven ion guides 9, ions pass through a differential pumping system (which includes vacuum chambers 1, 2 and 3 and lens/skimmer 8) before entering the high vacuum region 1 wherein the mass analyzer (not shown) resides. Once in the mass analyzer, the ions are mass analyzed to produce a mass spectrum.
Many other ion production methods might be used at atmospheric or elevated pressure. For example, MALDI has recently been adapted by Victor Laiko and Alma Burlingame to work at atmospheric pressure (Atmospheric Pressure Matrix Assisted Laser Desorption Ionization, poster #1121, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998) and by Standing et al. at elevated pressures (Time of Flight Mass Spectrometry of Biomolecules with Orthogonal Injection+, Collisional Cooling, poster #1272, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25-29, 1998; and Orthogonal Injection TOFMS Anal. Chem. 71(13), 452A (1999)). The benefit of adapting ion sources in this manner is that the ion optics and mass spectral results are largely independent of the ion production method used.
An elevated pressure ion source always has an ion production region (wherein ions are produced) and an ion transfer region (wherein ions are transferred through differential pumping stages and into the mass analyzer). The ion production region is at an elevated pressurexe2x80x94most often atmospheric pressurexe2x80x94with respect to the analyzer. The ion production region will often include an ionization xe2x80x9cchamberxe2x80x9d. In an ESI source, for example, liquid samples are xe2x80x9csprayedxe2x80x9d into the xe2x80x9cchamberxe2x80x9d to form ions.
The design of the ionization chamber used in conjunction with atmospheric pressure ionization mass spectrometry (API-MS) has had a significant impact on the availability and use of these ionization methods with MS. Prior art ionization chambers are inflexible to the extent that a given ionization chamber can be used readily with only a single ionization method and a fixed configuration of sprayers. For example, in order to change from a simple electrospray method to a nano electrospray method of ionization, one had to remove the electrospray ionization chamber from the source and replace it with a nano electrospray chamber (see also, Gourley et al. U.S. Pat. No. 5,753,910 (Gourley et al.), entitled Angled Chamber Seal for Atmospheric Pressure Ionization Mass Spectrometry).
The ion transfer region will generally include a multipole RF ion guide. Ion guides have been shown to be effective in cooling ions and in transferring them from one pressure region to another in a differentially pumped system. For example, ions may be produced by ESI or APCI at substantially atmospheric pressure. These ions are transferred from atmospheric pressure to a first differential pumping region by the gas flow through a glass capillary. Ions are directed from this first pumping region to a second pumping region by an electric field and by gas flow through a xe2x80x9cskimmerxe2x80x9d. A multipole in the second differentially pumped region accepts the ions and guides them through a restriction and into a third differentially pumped region. Meanwhile, collisions with gas flowing through the multipole xe2x80x9ccoolsxe2x80x9d the ions resulting in both more efficient ion transfer and the formation of a cool ion beamxe2x80x94which is more readily mass analyzed.
Depicted in FIG. 2 is a prior art ion source as described in Whitehouse et al. U.S. Pat. No. 5,652,427 (Whitehouse et al.). As discussed above with respect to FIG. 1, ions are formed from sample solution by an electrospray process when a potential is applied between sprayer 12 and sampling orifice 13. According to this prior art design shown in FIG. 2, a capillary is used to transport ions from atmospheric pressure where the ions are formed to a first pumping region 53. Lenses 47, 51, and 53xe2x80x2 are used to guide the ions from the exit of the capillary 50 to the mass analyzer 57 in the mass analysis region 54xe2x80x94in this case a quadrupole mass analyzer. Between lenses 47 and 53xe2x80x2, an RF only hexapole ion guide 40 is used to guide ions through differential pumping stages 41 and 42 to exit 52 and into mass analysis region 54 through orifice 47. The hexapole ion guide 40, according to this prior art design, is intended to provide forth efficient transport of ions from one locationxe2x80x94i.e. the entrance 48 of lens/skimmer 47xe2x80x94to a second locationxe2x80x94i.e. exit 52. Further, through collisions with rest gas in the hexapole, ions are cooled to thermal velocities.
In the scheme of Whitehouse et al., an RF only potential is applied to the multipole. As a result, the multipole is not xe2x80x9cselectivexe2x80x9d but rather transmits ions over a broad range of mass-to-charge (m/z) ratios. Such a range as provided by a prior art multipole is adequate for many applications, however, for some applicationsxe2x80x94particularly with MALDIxe2x80x94the ions produced may be well out of this range. High m/z ions such as are often produced by the MALDI ionization method are often out of the range of transmission of prior art multipoles.
In other schemes a multipole might be used to guide ions of a selected m/z through the transfer region. For example, Morris et al., in H. R. Morris et al., High Sensitivity Collisionally-Activated Decomposition Tandem Mass Spectrometry on a Novel Quadrupole/Orthogonal-Acceleration Time-of-Flight Mass Spectrometer, Rapid Commun. Mass Spectrom. 10, 889 (1996), use a series of multipoles in their design. One of these is a quadrupole. The quadrupole can be run in a xe2x80x9cwide bandpassxe2x80x9d mode or a xe2x80x9cnarrow bandpassxe2x80x9d mode. In the wide bandpass mode, an RF-only potential is applied to the quadrupole and ions of a relatively broad range of m/z values are transmitted. In narrow bandpass mode both RF and DC potentials are applied to the quadrupole such that ions of only a narrow range of m/z values are selected for transmission through the quadrupole. In subsequent multipoles, the selected ions may be activated towards dissociation. In this way the instrument of Morris et al. is able to perform MS/MS with the first mass analysis and subsequent fragmentation occurring in what would otherwise be simply a set of multipole ion guides.
FIG. 3 depicts such a prior art source design according to Morris et al. This prior art design is similar to that of Whitehouse et al. (as shown in FIG. 2), except that the multipole source design according to Morris et al., four RF multipoles (i.e., 14-17) are used. The first multipole encountered by the ions is hexapole 14. It is used in a manner similar to the design of Whitehouse et al. to cool and guide the ions. The second multipole encountered is quadrupole 15. Quadrupole 15 can be used in a wide bandpass mode, to transmit ions over a broad m/z range, or in a narrow bandpass mode, to transmit ions of a selected narrow m/z range. This leads to the use of the mass spectrometer instrument 10 in MS and MS/MS modes. In MS mode, quadrupole 15 is operated as a wide bandpass ion guide. Ions are simply transmitted by all four multipoles 14-17 to time-of-flight (TOF) mass analyzer 18. The TOF mass analyzer is then used to produce a mass spectrum. In MS/MS mode, quadrupole 15 is operated as a narrow bandpass ion guide to select ions of interest. Further, the third multipolexe2x80x94hexapole 16xe2x80x94is operated with a DC offset with respect to quadrupole 15 and is filled with a collision gas. This leads to collisions between the ions of interest and the collision gas and can result in the formation of fragment ions. The fragment ions are guided by hexapole 17 to TOF analyzer 18 which is then used to produce a mass spectrum of these fragment ions.
However, the prior art design of Morris et al., when used in xe2x80x9cwide bandpassxe2x80x9d mode, is unable to transmit as wide an m/z range as that of Whitehouse et al. described above and certainly not as high an m/z as ions produced by MALDI. The Whitehouse et al. design uses a hexapole. Other prior art designs use an octapole or even a pentapole as the ion guide. Hexapoles, octapoles, and pentapoles are not as good as the Morris design for m/z selection. However, the quadrupole (used in the Morris design) cannot transmit as wide an m/z range as a hexapole, octapole, or pentapole. While some prior art multipoles might be better for transmitting ions of a broad m/z range and others might be better for ion selection, none can transmit high m/z ions such as produced in MALDI (m/z less than xcx9c105 Th) (mass-to-charge ratio is less than approximately 105 Thompsons).
The purpose of the present invention is to provide an improved ionization source chamber and ion beam delivery system for use with mass spectrometers. It is a further purpose of the present invention to provide a means and method of operating a mass spectrometer which uses such an ionization source chamber and ion beam delivery system to provide ions to the analyzer and analyze them in a mass analyzer. It is yet a further purpose of the present invention to provide a means and method of operating a mass spectrometer which utilizes the ionization source chamber and ion beam delivery system with a variety of ionization techniques (i.e., ESI, MALDI, etc.).
One aspect of the present invention is to provide an ionization source chamber and ion beam delivery system which has improved flexibility over prior art sources. The ionization source chamber and ion beam delivery system according to the present invention includes a port onto which an ion production means can be mounted. A variety of ion production meansxe2x80x94including electrospray ionization and matrix assisted laser desorption/ionizationxe2x80x94may be used. Each ion production means is integrated onto its own flange. To select the desired ion production method, the flange including the means for that particular method is mounted on the port of the ion source.
According to another aspect of the invention, a means is provided whereby one can easily obtain access to the ion transfer optics in an elevated pressure ionization source chamber and ion beam delivery system. That is, a flange can be openedxe2x80x94without demounting any hardware or supporting electronicsxe2x80x94to provide easy access to electrodes of the ion transfer optics which need regular cleaning.
Other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, all of which form a part of this specification.